The present invention relates generally to a method for determining the roll angle of a moving vehicle. More particularly, the present invention relates to a method for detecting the roll angle under dynamic lateral operating conditions for use in a motor vehicle having a yaw or rollover control system.
Automotive vehicles with braking systems which respond to vehicle conditions as well as driver input have been produced. For example, when a particular yaw rate is desired, as indicated by a driver""s steering wheel operation, if the vehicle is not producing an adequate yaw rate, the braking system of the vehicle may compensate by altering a particular wheel""s speed. This control is dependent on accurate measurement of several vehicle operating conditions. It has been observed that error can be introduced in the control system if the vehicle is operating on a banked surface. Therefore, it is desirable to determine the bias in the various operating condition signals introduced by operating on a banked surface.
U.S. Pat. No. 5,446,658 (""658) addresses the problem of estimating bank angles of a road surface. However, under various operating conditions, the system described in ""658 patent does not have the ability to calculate the road surface bank angle under dynamic lateral operating conditions. Specifically, if the yaw rate for the vehicle changes by more than a predetermined threshold, the previously determined bank angle is assumed as the current bank angle. Of course, it is quite possible for a vehicle to undergo extreme bank angle variation during the period that the yaw rate is not within a predetermined threshold.
U.S. Pat. No. 6,073,065 provides a method for determining a bank angle experienced by a motor vehicle that is robust to dynamic lateral vehicle operations. However, the methodology reaches its limitation during high frequency maneuvers with simultaneous road bank variation. The estimation during dynamic maneuvers may experience some instantaneous errors.
A paper presented by Y. Fukada entitled xe2x80x9cEstimation of Vehicle Slip-Angle With Combination Method of Model Observer and Direct Integrationxe2x80x9d, presented at the International Symposium on Advance Vehicle Control in September 1998, addresses the estimation of road bank angle with vehicle slip angle. In this paper, the difference between lateral force measured with a lateral accelerometer and the tire model estimated lateral force is used to obtain the road bank angle. However, its accuracy is contingent on the accuracy of the estimated lateral tire force, which, in turn depends on the accuracy of the road surfaces and lateral velocity. In this approach, the lateral velocity in road bank angle estimation is believed to be, at best, a convoluted process. Also, the accuracy and robustness of such a calculation is suspect.
In roll sensing systems, three accelerometers and three angular rate sensors have been used. Together the sensors track the position and attitude of the vehicle. One problem with such systems is that the output of the sensors is susceptible to cumulative drift errors. Some gyroscopic sensors are available to minimize drift but are typically unsuitable and too expensive for automotive applications. U.S. Pat. Nos. 6,038,495 and 6,212,455 use automotive grade sensors to determine rollover of the vehicle. Such systems take advantage of short-term integration and/or an inclinometer for bias correction. However, such systems are not capable of detecting sustained road surfaces such as the road bank during a turn. Under bank conditions, the frame of reference changes and therefore the output does not provide a reliable vehicle roll attitude. The ""495 patent, for example, uses integration with a high pass filter while the ""455 patent uses a lateral accelerometer/inclinometer to provide long term offset compensation. During bank turn conditions, the steady state roll is no longer zero and the lateral acceleration is not a good indication of the vehicle roll angle. Therefore, neither of these approaches provides robust roll angle estimation for road disturbances such as road bank angles, and in particular, sustained road bank angles.
It would be desirable to determine roll angle under a variety of conditions including a bank angle being experienced by the vehicle.
The present invention provides a robust method and apparatus for determining the roll angle of a vehicle that takes into consideration the suspension roll, the road bank angle experienced by a motor vehicle, as well as sensor measurement drift errors. The method comprises providing a lateral acceleration signal corresponding to a lateral acceleration of the vehicle, providing a yaw rate signal corresponding to a yaw rate of the vehicle, providing a steering wheel signal responsive to a sensed steering wheel angle of the vehicle, providing a speed signal corresponding to a vehicle speed, calculating a first bank angle estimate dependent on the lateral acceleration signal, the yaw rate signal, the steering wheel signal and the speed signal, calculating a suspension reference roll angle in response to said lateral acceleration signal, calculating a conservative bank angle estimate signal in response to the first bank angle estimate and the suspension roll reference angle.
In a further aspect of the invention where the total vehicle roll/bank angle is estimated, a system for controlling a vehicle includes a lateral acceleration sensor generating a lateral acceleration signal corresponding to a lateral acceleration of the vehicle, a roll rate sensor generating a roll rate signal corresponding to a roll rate of the vehicle, a yaw rate sensor generating a yaw rate signal corresponding to a yaw rate of the vehicle, a steering wheel angle sensor generating a steering wheel angle signal corresponding to a steering wheel angle of the vehicle, a speed sensor generating a speed signal corresponding to the longitudinal speed of the vehicle. A controller is coupled to the lateral acceleration sensor, the yaw rate sensor, the steering wheel angle sensor and the speed sensor. The controller calculates a first roll angle signal in response to the roll rate signal and calculates a second roll angle signal corresponding to a suspension reference roll angle in response to the lateral acceleration signal. The controller calculates a bank angle signal in response to the lateral acceleration signal, the yaw rate signal, the steering wheel signal and the speed signal. The controller sums the first roll angle signal, the second roll angle signal and the bank angle signal to obtain a final roll angle estimate.
One advantage of the present invention is that drift of the measured signal is reduced so that the present invention is more accurate in relatively long banked turns.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.